Summary

The world's rivers are regulated by about 58,000 large dams (more than 15 m high) that provide water supplies for municipalities and irrigation, allow downstream navigation, and enable hydropower production (1). New dams are widely seen as sources of green energy. An estimated 75% of the world's potential hydropower capacity is unexploited (2), and some 3700 new dams are currently proposed in developing economies (3, 4). But dams also cause substantial and often unacknowledged environmental damage. Recent research affords insight into how dams might be strategically operated to partially restore some lost ecosystem functions and services.